Editorial: A spectacle to inspire engineers of the future
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Strapped into a custom built seat, Andy Green prepares for the ride of his life. The pancake-flat desert stretches out for miles ahead. The computer indicates all systems are normal. He eases off the brakes and puts his foot down on the throttle. The jet engine roars into life. In precisely 42.5 seconds he’ll be travelling 1000 mph. In a car.
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“It’s almost impossible to tell the difference between going supersonic in a car and in an aircraft,” says Green. He is the only person on Earth who can say that from personal experience. Green was a fighter pilot for the UK Royal Air Force for 20 years, and he is also the fastest man on wheels. In 1997, driving a vehicle called ThrustSSC, he set the world land speed record of 763 miles per hour, becoming the first and only person to break the sound barrier in a car (761 mph under standard conditions). Now, together with the Bloodhound SSC design team, he’s attempting to do it all over again, and then some.
This time there’s competition. A three-way race is developing, with two other teams, one from North America and the other from Australia, vying to wrest the record from the Brits. The first step will be to break the existing record and get past 800 mph. If that succeeds, the next stage is to attempt 1000 mph (1609 kilometres per hour). “That’s what we’re designing the car for,” says Ron Ayers, chief aeronautic engineer on the Bloodhound project.
All three competing vehicles have wheels, brakes and a steering wheel, but that’s pretty much where the similarity with conventional cars ends. Getting up to the speed of sound and beyond poses challenges that a normal car will never encounter, requiring some radical design and engineering.
For example, the wheels of a 1000 mph car will need to rotate at over 10,000 revolutions per minute, many times faster than on an ordinary car. This rate of spin entails an acceleration of almost 50,000 g at the rim, generating forces that would easily tear conventional wheels apart. Instead, this car will need wheels of solid titanium, or more likely carbon-reinforced aluminium. What’s more, as the vehicle approaches the speed of sound, it produces a frontal shock wave which fluidises the earth ahead, so the wheels end up carving through ground, rather than simply rolling over it. On top of all that, beyond 250 mph, airflow starts to become a more important consideration in controlling the vehicle than traction on the ground. At this speed, the wheels begin to behave like rudders or aerofoils, and driving the car becomes more like controlling a speedboat or flying an aircraft. “Our biggest single concern is to make sure the vehicle stays on the ground,” says Green.
“As the car approaches the speed of sound, it produces a shock wave which fluidises the earth ahead”
Creating a “car” that takes account of all these factors means exploring unchartered territory in aerodynamics and vehicle mechanics. “No one has ever designed a car to go this fast before,” says Green, “so we’ve got to develop and test, develop and test… it’s an ongoing research project.”
Time = 10 seconds
Green hits 79 mph. At this stage he would be eating the dust of an average sports car, but Bloodhound is an automotive wolf in sheep’s clothing. Green holds steady, and 5 seconds later unleashes the first of his secret weapons: an afterburner which dumps extra fuel into the jet engine, stoking it up to full power.
Bloodhound SSC will use a retired Eurofighter jet engine to provide the first-stage thrust for the car. In that respect it resembles ThrustSSC, which was powered entirely by two jet engines. But according to Ayers, that set-up won’t be good enough to reach 1000 mph. “The large front inlets [for the jet engines to take in air] on ThrustSSC produced huge shock waves at supersonic speed,” he says. “This means we couldn’t get any more than a 5 per cent increase in speed using that design.”
The joint US-Canadian team, North American Eagle, begs to differ. They are going entirely with jets. Instead of designing a car from scratch, they have taken the fuselage of a scrapped F-104 Starfighter aircraft, added the engine from an F-4 Phantom supersonic fighter-bomber bought from a surplus seller, and bolted on some wheels. “We know the aircraft can do around 1500 mph, so if we can do just half of that on land we’re already pretty close to the record,” says Ed Shadle, the car’s driver and co-owner.
Unlike their competitors, North American Eagle is already built and rolling. Shadle has done 27 runs so far, pushing the car to 400 mph to test the parachute systems and brakes, and to collect data to model what will happen at higher speeds. He is now refining the wheels and aerodynamics. “We’re hoping to go after the record on the Fourth of July 2010,” he says.
As yet he doesn’t know where that attempt will be made, because his decision to go with jets alone has presented him with a very difficult problem: the sheer length of track he will need to accelerate to 1000 mph and then decelerate to a stop. The terrain has to be as flat as glass; any bumps might send the car off-track with disastrous consequences. Shadle is looking for a site with around 14 miles of uninterrupted flatness. Ayers and the Bloodhound team aren’t convinced that will be easy to find.
Their approach is to shorten the run as much as possible by going for more thrust. They are designing for a 10-mile run, a constraint that has led them to make a radical choice: instead of two jet engines, they will use one jet for the initial acceleration and then boost to full speed with a rocket.
Building both types of engine into the car has been no mean feat. The team initially wanted the rocket to sit above the jet engine, as this would lower the centre of gravity of the car and make it more stable. But this configuration led to the risk of oversteer, so they reluctantly swapped the engines around.
It may seem a trivial decision, but the change has far-reaching consequences. It alters not only the internal design of the car but also the aerodynamics, and it dramatically affects the distribution of forces across the front and back wheels, presenting a whole new set of problems. But control ultimately governs the top speed, so jet over rocket is their only option.
Time = 23 seconds
Travelling at 269 mph, Bloodhound is now speeding faster than a Formula 1 car at top whack. Green braces himself, then pushes a button that fires the rocket. In an instant he is pushed back in his seat as the car’s acceleration ramps up to 2.3 g.
Rocket powered cars are not a common sight even in land-speed record attempts, because the thrust from a rocket is so huge, rapid and hard to control. That’s why just about the only vehicles powered by rockets are drag cars.
Huge, rapid and hard to control thrust is no bother to Rosco McGlashan, an Australian drag racer who has set up a rival team to Bloodhound and North American Eagle. His car, Aussie Invader, will also be propelled by rockets – in fact, nothing but rockets.
“I think the Bloodhound project is a great idea, but it’s too complicated,” he says. “I believe in keeping it as simple as possible. An all-rocket design is as simple as it gets.” If all goes to plan, Aussie Invader will reach the 1000 mph target thanks to four rockets which will give it more than 200 times the power of a Formula 1 car.
Rocket-based cars do have one significant downside, however: the exhaust plume is so fierce that it is likely to excavate a ditch behind the car. That’s not a problem for a one-way journey. To meet the official requirements for the record, however, a car has to not only cover 1 mile in under 3.6 seconds, but also repeat the feat in the opposite direction along a parallel track within the hour. If the car veers off course for any reason, the last thing the driver wants to have to contend with is a nearby ditch.
Time = 42.5 seconds
4.2 miles down the track, Green hits 1000 mph. In less than 2 seconds his official 1-mile run will start; 3.5 seconds later it will be over and he could be halfway to setting a new record.
With all that rocket power at its disposal, the Australian car is the hare to Bloodhound’s tortoise. “We reach 1000 mph in 19 seconds,” says McGlashan. That’s over twice as fast as Bloodhound, meaning McGlashan will reach top speed just 3 miles down the track.
Rocket-powered cars come with other problems, though. For a start, the rate at which they burn fuel is enormous. Aussie Invader will consume almost 3 tonnes of fuel and oxidiser to reach top speed, which is delivered through a system of pressurised gas cylinders. Though Bloodhound’s fuel consumption is lower, its rocket still requires fuel to be delivered at a phenomenal rate. It achieves this thanks to a 4.2-litre internal-combustion engine whose sole function is to keep the oxidant flowing in.
With all this power and speed, you might wonder what safety systems have been built in. The answer: surprisingly little. All the cockpits are reinforced, and there are seat belts and emergency kill switches for the engines, but none of the three cars has an ejector seat. According to Green, this is “by far the safest” arrangement. “Designing an ejector system is a multi-million-pound project in its own right that could be fraught with problems itself. If we can keep the wheels on the ground, why would you want to leave the car?”
“None of the three cars has an ejector seat. This is by far the safest arrangement”
For McGlashan, the local wildlife probably represent the greatest threat. “Imagine a kangaroo running across the track while you’re travelling at 1000 mph. They could appear from out of nowhere – that’s probably the biggest danger in Australia,” he says.
“Imagine a kangaroo running across the track while you’re travelling at 1000 mph”
Then there is the minor issue of stopping.
Time = 47.6 seconds
Green completes the measured mile, cuts the rocket and jet, and is thrown forward in his seat as the car lurches from 2 g of acceleration to 3 g of deceleration. In 9 seconds the first parachute will be deployed, followed 7 seconds later by a second. Then it will be time to slam on the brakes.
It won’t be that straightforward for McGlashan. “We can’t just flick a switch and kill the engines or we’ll get 16 g of deceleration,” he says. That would put him at serious risk of injury: even highly trained fighter pilots like Green pull no more than 12 g during aerobatic manoeuvres.
So McGlashan has to turn the rockets off in stages. Two rockets are automatically cut as he enters his measured mile, the other two around 2 seconds later. At this point he is still moving too fast to fire a parachute, so he deploys a “stinger” – a 50-metre metal cable that creates plenty of air drag as it trails behind the car, while helping to keep the car in a straight line. Three seconds later his speed should have fallen to around 700 mph and he can deploy a chute. “When I hit 500 mph, it’s time to lean hard on the carbon-fibre brakes”, he says. Those brakes will burn up and need replacing at the end of the run.
To find a suitably long and flat track for the Bloodhound team, Green has been to Turkey, the US, Australia and South Africa to check out salt pans and mud flats. The two different surfaces each have pros and cons. Salt pans are unforgivingly hard and slippery, but tend to be glassy flat and clear of debris. Mud flats are more forgiving but they are often covered in rocks or stones. “We will need to sweep an area 18 kilometres by 500 metres to clear enough space for the tracks,” says Green.
Mud flats pose another unexpected problem, says Ayers. The supersonic shock wave throws up a spray of dust, altering the flow of air around the car and creating “spray drag”. McGlashan points out that the dust might present an additional problem for the air-breathing jet engines. “With a rocket you don’t have an air intake, so you don’t have that problem.”
McGlashan is exploring a radical solution to the problem. “We’re talking to some people in Dubai who have suggested building a dedicated track for us.” Whether the money to do it is made available remains to be seen.
So who is going to be first to have a shot at the record? Green is by far the most experienced when it comes to supersonic cars, and his team has the most technical prowess. But while Bloodhound’s design is progressing apace, they don’t expect to make their first attempt for at least another 18 months. North American Eagle is up and running, and McGlashan has finished building the body of Aussie Invader and is just awaiting delivery of the rockets. “I don’t want to blow smoke up anyone’s bum, but with luck we might make an attempt in 12 months,” says McGlashan.
Time = 95.8 seconds
Green pulls to a halt. He has covered nearly 10 miles in just over 90 seconds. His average speed over the measured mile is 1013 mph. The record is there for the taking. All he has to do is turn around and do it all over again.
Editorial: A spectacle to inspire engineers of the future
